There are no VALID studies (and by "valid" I mean studies that were not funded by the ONDCP, LEO organizations, or similar.) to show that weed has detrimental effects other than
a. the obvious problems associated with inhaling smoke (of any kind)
b. some short-term memory loss associated with daily use of the substance.
Anything else you have heard probably came from the main-stream media hype buying into the extremely slanted studies funded by various United States agencies that have a vested interest in maintaining prohibition (e.g. ONDCP (Office for the national drug control policy, DEA (duh!), various police associations across the country, etc.).
Interestingly enough when you look at the studies done by groups or institutions that had nothing to lose either way (most of these are in OTHER countries than the US cause it's near impossible to get funding without strings regarding this subject), the effects are near minimal (long-term effects amount to a. and b.).
Consider the reality that weed is likely the OLDEST known recreational drug known to man. There has NEVER been a recorded case in history where someone died from over-use in any way.
I used to default to the opinion that "weed was bad" and you'd have to be an idiot to smoke it (in my youth I would have been quite the idiot, then I grew up and quit and adopted the propaganda). Lately however, being "Mr. Science" I decided to actually do a bit of research on the subject (reading, NOT smoking here guys!! I still don't smoke the stuff). When you start to read real information and consider the "facts" I'd worry about smoking dope about as much as I'd worry about having a beer after work.
So don't sweat it. Smoke em if you've got em. Just don't abuse them, or anything for that matter (i.e. booze, real drugs, sugar, carbs, sex...er, no wait that one's O.K.!)